I'm not sure if this is an option for your application, but back in the day I hardware boxxed 6 toons in DAOC. 6 monitors wasn't an option (especially since LCD's weren't bargain basement like they are now - CRT's all around), I could realistically only have 3 monitors.

I used 1 monitor for my main (bard), another for my healer (druid) and a 3rd monitor for the other 4 toons (animists) - but I had all the machines connected to the monitor via KVM. As long as your control of your slaves are solid, you have no problem.

The multiboxer motto in DAOC was "I don't need to see them to control them".